This invention relates to integrated circuit power up, more particularly, a circuit to detect predetermined power supply levels so that sufficient power is provided for an integrated circuit to function properly and drive a bus, the circuit operating in a low power mode when predetermined power supply levels are detected.
Computer systems typically include one or more processors. A processor manipulates and controls the flow of data in a computer. Typically, if a processor fails, the computer system fails. Processor failure may occur due to, for example, insufficient power being provided by one or more power supplies. That is, the magnitude of supply voltage can be below the minimum nominal operating voltage required by the processor.
Processors are presently designed to consume and require minimal power. When the Merced multiprocessor system powers up, voltage is supplied to each of the different integrated circuits on the bus. Since the Merced processor has dual power supplies, a core power supply having 1.1 volts and an input/output (I/O) power supply having 1.5 volts, it is necessary to ensure that both power supplies reach functional levels before enabling any Front Side Bus (FSB) functionality. If the voltage supplied to the Merced integrated circuit is insufficient for the integrated circuit to function properly, the integrated circuit must not drive the FSB because it would prevent other integrated circuits on the system and the system itself from functioning properly. A faulty core power supply could improperly drive the FSB pins, and a faulty I/O power supply could result in faulty integrated circuit behavior regardless of the state of the integrated circuit core. Although voltage sensors exist for detecting sufficient power supplies in the Deschutes phase locked loop (PLL) and in the Merced PLL, their low power modes (IDDQ) can result in sufficient power being incorrectly indicated while the core supply is inadequate. This invention ensures that power supplies, including both the core power supply and the I/O power supply reach sufficient levels before the Merced integrated circuit drives the FSB. The invention teaches disabling FSB pin driving until sufficient core power is detected, the detection provided by a circuit that can be disabled for low power operation when sufficient core power and I/O power is detected, and the circuit designed to meet high volume manufacturing standards.